1. Field of the Invention
This invention relates to an ultrasonic detection apparatus, and to a method for detecting ultrasonic vibration or acoustic emission (AE) produced by a high-speed rotating member in a non-contact manner.
2. Description of the Related Art
A rotary dressing apparatus has been used to shape a grinding wheel (stone) for grinding workpieces. This type of rotary dressing apparatus generally includes a housing, a dresser shaft which is rotatably supported in the housing, and a dresser which has a cutting edge of diamonds or the like at the tip portion thereof and is mounted to the end portion of the dresser shaft. In use, the dresser is contacted with a rotating grinding wheel while rotating the dresser shaft to thereby shape the surface of the grinding wheel.
In order to enhance the efficiency of a shaping process, it is required to minimize contact (grinding allowance) between the dresser and the grinding wheel. This requirement is somewhat satisfied by an operator relying on his hearing to detect the noise produced when the dresser wheel contacts the grinding wheel. However, such human detection is low in accuracy and is generally accompanied by substantial error.
In order to enhance the grinding-wheel shaping precision, reduce loss of abrasive grains, improve the use efficiency of the grinding wheel and reduce the grinding cost, the contact between the dresser and the grinding wheel is required to be automatically detected in an electrical or mechanical manner. In order to satisfy this requirement, for example, U.S. Pat. No. 4,744,348 (Japanese Utility Model publication No. 64-278) discloses an ultrasonic detection apparatus having an ultrasonic or AE sensor for detecting ultrasonic vibrations or acoustic emissions produced when the dresser is contacted with the grinding wheel, thereby automatically detecting the contact between the dresser and the grinding wheel without human detection.
In the ultrasonic detection apparatus as disclosed in these publications (as shown in FIG. 10), an AE sensor 61 is provided integrally with a housing (supporter) 63 for supporting a dresser 62 serving as a rotator, and a cutting (detection) liquid is supplied from a detection liquid supply unit 64 into a gap between the rotating dresser 62 and the housing 63 to form a liquid membrane in the gap. The ultrasonic vibration (AE) produced when the cutting edge 65 of the dresser 62 is contacted with the grinding wheel is transmitted through the liquid membrane and the housing 63 to the AE sensor 61 to detect the contact between the dresser and the grinding wheel with a resolution of in the micron range.
FIG. 11 shows a cross-sectional view of the ultrasonic detection apparatus shown in FIG. 10, and shows an arrangement of the detection liquid supply unit 64. As shown in FIG. 11, the detection liquid supply unit 64 for supplying the detection liquid into the gap between the rotating dresser 62 and the housing 63 is disposed away from the rotational axis of the dresser, and it supplies the detection liquid from such a position into the gap. Therefore, the liquid membrane 66 is radially spread around the rotational axis in a doughnut shape. When the dresser 62 rotates at a high speed, the doughnut-shaped liquid membrane 66 is greatly scattered from the peripheral surface of the dresser 62 due to centrifugal force, and thus the liquid membrane cannot be evenly formed in the gap. Therefore, the transmission of the ultrasonic vibration through the liquid membrane to the AE sensor 61 is disturbed, and thus high precision cannot be obtained for detection of the ultrasonic vibrations as shown in FIG. 9.
FIG. 9 is a graph showing a measurement result of an output N with respect to a supply voltage S when the rotator was rotated at a high speed of 5000 rpm. In FIG. 9, the abscissa represents time SEC in seconds and the ordinate represents an effective voltage V (volt). An S/N was calculated on the basis of the above measurement result to be 3.5. In general, the detection sensitivity of the AE sensor is good when the S/N exceeds 5, and thus the conventional ultrasonic detection apparatus as described above has a low sensitivity.